1. Field of the Invention
The present invention relates generally to a communication system using a multi-antenna, and in particular, to an apparatus and method of enhancing decoding performance at a receiver using an Iterative Detection and Decoding (IDD) scheme.
2. Description of the Related Art
Recently, with rapid growth of the wireless mobile communication market, various multimedia services in the wireless environment are becoming more heavily demanded. In particular, mass transmission data and rapid data delivery are progressing. Thus, an urgent task is to find a method of efficiently using limited frequencies. To respond to this, a new transmission technique using a multi-antenna is desired. By way of example of the new transmission technique, a Multiple Input Multiple Output (MIMO) system using a multi-antenna is being used.
A MIMO technique, which uses a multi-antenna at a transmitter and a receiver respectively, can increase the channel transmission capacity in proportion to the number of the antennas without additional frequencies or transmit power allocation, compared to a system using a single antenna. Thus, in recent years, active research has been conducted on the MIMO technique.
Multi-antenna techniques are divided largely to a spatial diversity scheme which improves the transmission reliability by acquiring a diversity gain corresponding to the product of the numbers of transmit and receive antennas, a Spatial Multiplexing (SM) scheme which increases the data rate by transmitting a plurality of signal streams at the same time, and a combination scheme of the spatial diversity and the SM schemes.
When adopting the multi-antenna technique, it is known that a Maximum Likelihood (ML) receiver is an optimum scheme to detect a receive signal. However, it is hard to practically realize a ML receiver because complexity increases by raising a length of a codeword to the power of the transmit antennas. Hence, active researches are being conducted on a reception scheme which obtains low complexity and performance close to the ML receiver.
In the mean time, recently, an IDD scheme which applies a turbo principle to a MIMO receiver is attracting great attention. An IDD scheme regards the MIMO part as a coder and models it by concatenating a channel coder with a MIMO coder. In more detail, an IDD scheme is an iterative decoding scheme which inputs the output of a MIMO detector to a channel decoder and feeds back the output of the channel decoder to the MIMO detector. Simply, the MIMO detector generates soft decision information for the receive signal and provides the soft decision information to the channel decoder, and the channel decoder recalculates soft decision values for the respective bits using the input soft decision information as priori information. The soft decision values calculated at the channel decoder are fed back to the MIMO detector and then used as priori information at the MIMO detector. By repeating this process, reliability of the received bits is enhanced.
Representative schemes of a conventional MIMO IDD include List MIMO [B. M. Hochwald and S. ten Brink, “Achieving near-capacity on multiple-antenna channel”, IEEE Trans. on Commun, vol. 51, pp. 389-399, March 2003] and turbo-BLAST (Vertical Bell Labs Layered Space-time) [M. Sellathurai and S. Haykin, “TURBO-BLAST for wireless communication: theory and experiments”, IEEE Trans. on Signal Processing, vol. 50, pp. 2538-2546, October 2002]. Those two schemes are the same as the IDD scheme, except for the spatial multiplexing at the transmitter and the MIMO signal detection at the receiver. Hereinafter, descriptions are centered on the List MIMO scheme which is known for its good performance.
First, a system model according to the prior art is defined. As shown in FIG. 3, a system of interest similar to the present invention includes NT-ary transmit antennas and NR-ary receive antennas.
A signal vector d consisting of K-ary bits to transmit is transformed to a signal vector c consisting of N-ary coded bits at a channel coder and interleaved at an interleaver having the size N. The interleaved signal vector {tilde over (c)} is transmitted on the plurality of the antennas via a MIMO transmitter (modulator and demultiplexer). Provided that M-ary coded bits are mapped to a modulation symbol, the number of the coded bits transmitted on the plurality of the antennas at a time is NT×M. That is, to transmit all of the N-ary coded bits, transmission as many as N/NTM times are required.
Given the transmit vector at the m-th transmission expressed as x(m)=[x1(m)x2(m) . . . xNT(m)]T, m=1, . . . , N/NTM, the receive vector y(m)=[y1(m)y2(m) . . . yNR(m)]T received at the receiver is expressed as Equation (1).y(m)=H(m)x(m)+n(m)  (1)
In Equation (1), n(m) denotes noise vector NR×1. H(m) is a NT×NR matrix formed between the transmit and receive antennas at the m-th transmission and can be expressed as Equation (2).
                    H        =                  [                                                                      h                  11                                                                              h                  12                                                            …                                                              h                                      1                    ⁢                                          N                      R                                                                                                                                            h                  21                                                                              h                  22                                                            …                                                              h                                      2                    ⁢                                          N                      R                                                                                                                          ⋮                                            ⋮                                            …                                            ⋮                                                                                      h                                                            N                      T                                        ⁢                    1                                                                                                h                                                            N                      T                                        ⁢                    2                                                                              …                                                              h                                                            N                      T                                        ⁢                                          N                      R                                                                                                    ]                                    (        2        )            
In Equation (2), the element hij corresponding to the i-th line and the j-th column denotes a channel response at the i-th transmit antenna with the j-th receive antenna.
A conventional IDD receiver for receiving signals transmitted on the plurality of the antennas is now described.
FIG. 1 shows a conventional IDD receiver in a MIMO antenna system. The conventional IDD receiver includes a list generator 100, a MIMO detector 102, a first subtracter 104, a deinterleaver 106, a Soft-Input Soft-Output (SISO) decoder 108, a second subtracter 110, an interleaver 112, and a hard decision part 114.
The list generator 100 generates a list ^1 of a candidate coded symbol vectors c=[c1c2 . . . cNl·M] of high reliability using a sphere decoding or a Sorted-Modified ML (S-MML) with respect to the receive vector y. FIG. 2 depicts a list which is generated by selecting four coded symbol vectors in a coded symbol vector space.
The MIMO detector 102 generates a first posterior information vector LD1 by MIMO-decoding the receive vector y and the priori information from the interleaver 112. At this time, the MIMO detector 102 performs the decoding using the sphere decoding or the S-MML scheme, and the list of the candidate coded symbol vectors used for the decoding is provided from the list generator 100. At the first iteration, since there is no priori information for each bit, probabilities of being +1 and −1 are initialized to ½, respectively.
The first subtracter 104 produces a first extrinsic information vector LE1 by subtracting the first priori information vector LI1 of the interleaver 112 from the vector LD1 of the MIMO detector 102. The deinterleaver 106 generates a second priori information vector LI2 by deinterleaving the first extrinsic information vector LE1 from the first subtracter 104.
The SISO decoder 108 generates a second posterior information vector LD2 by SISO-decoding the second priori information vector LI2 from the deinterleaver 106. The second subtracter 110 generates a second extrinsic information vector LE2 by subtracting the second priori information vector LI2 of the deinterleaver 106 from the second posterior information vector LD2 of the SISO decoder 108. The interleaver 112 generates the first priori information vector LI1 by interleaving the second extrinsic information vector LE2 from the second subtracter 110.
After repeating the process for a number of times, at the last iteration, the SISO decoder 108 outputs Log-Likelihood Ratios (LLRs) (LD2,information) corresponding to the K-ary information bits to the hard decision part 114. Next, the hard decision part 114 generates information bits through the hard decision on the fed LLRs.
In the conventional LIST MIMO scheme of FIG. 1, once calculated, the initial list ^ is fixed and then used the same at all IDD iterations. However, since the LLRs generated at the SISO decoder 108 are more accurate information than the LLRs generated at the MIMO detector 102, it is needed to far more accurately update the list by use of the LLRs generated at the SISO decoder 108. If it is possible to update the list more closely to the receive signal, the overall decoding performance of the IDD receiver will be enhanced.